1. Field of the Invention
The present invention relates to a reducing digital image having multiple tone density values into a binary image.
2. Description of the Prior Art
The term "binarizing" means to reduce a multi-value digital signal into a two-value binary signal. When documents are transmitted using means such as a facsimile device, an image is processed by using optical means and the image is converted into a digital image having a density value for each of the multiplicity of pixels (i.e., piture elements). These digital pixels are then reduced into binary digital signals, using a threshold value.
However, because the threshold value is a fixed value, when such devices are used for non-stop scanning of documents in which there is a variance in the desity values caused by differences in the colors of the paper and the density of the characters, there has been a problem of loss of information caused by an inability to binarize accurately all the image information in the documents.
Methods that have been employed to solve this problem include use of a floating, instead of a fixed, threshold value for the binarization, with the floating threshold value being obtained based on the average value of the pixel density values around the pixel that is to be binarized.
FIG. 11 illustrates an example of a threshold value setting method according to the prior art. In FIG. 11, the horizontal axis is the quantized average value of the pixel density values around the pixel that is to be binarized using an 8-bit binary number (0-255), and the vertical axis is the threshold value, also represents as an 8-bit binary number. As shown in FIG. 11, the threshold value is a function that changes in accordance with the average pixel density value, even when processing on a continuous basis a multiplicity of documents in which there is a variance in density values, binarization can be carried out in accordance with the density value of each document.
However, one of the problems with the conventional method described above is that only parts at which there is a large difference between the density value of the pixels to be binarized and the density value of the surrounding pixels are binarized correctly, that is, the borderline portions of the alphanumeric and graphic characters, while at parts where the density value of the pixels to be binarized is of the same order as the density value of the surrounding pixels, pixels having a density value that correctly should be converted as black has been incorrectly converted as white, producing characters and patterns with voids and pronounced borders.
This will now be considered with respect to the binarization of a sample image of a check, with reference to FIG. 12 (a) which shows the part where the amount has been entered. The amount section and the surrounding paper of the check both have around the same low density value (i.e., black). When the threshold value setting method illustrated by FIG. 11 was used, because the threshold value at this part was so low the low-density-value amount section was converted into white instead of into the original black. The inability to binarize the image correctly produced the result shown in FIG. 12 (b), a binarized image in which it is difficult to distinguish the amount portion.
Another example of the type of problem caused by the inability to binarize accurately is shown in FIG. 13 (a). In this case, in the process of binarization symbols relating to weather conditions, specifically the filled bullet a representing rain and the semi-filled bullet b representing thunderstorms, have been transformed into the wrong symbols owing to the loss of the black filling.